Traumatic brain injury (TBI) remains a significant healthcare problem. With severe traumatic brain injury multiple forms of brain damage interact and contribute to morbidity. In contrast, with more mild to moderate TBI, overt brain damage is uncommon with diffuse axonal injury representing the dominant form of change and playing a major role in any ensuing morbidity. While in both the basic science and clinical setting there is an increased appreciation of the pathogenesis of traumatically-induced axonal injury, many questions remain regarding the precise initiation of the traumatically-induced axonal injury, its electrophysiological correlates, and its potential therapeutic targeting. Moreover, it is unknown if any related microvascular change could influence the progression of DAI and/or if repeated insults to the injured brain can further exacerbate any observed axonal/microvascular change, an issue of great significance in the field of sporting-related injury. Given the importance of these issues and their overall relevance for achieving a better understanding of TBI and its potential therapeutic management, we will address these concepts utilizing a new model of TBI in YFP- expressing mice. Using this animal model with mild TBI, we will follow those axonal changes ongoing within the neocortex, assessing their initiation and progression through modern bioimaging approaches that allows with precision, the detection of the initiating site of axonal injury wherein we can discern its associated pathogenesis. These changes will be followed in vivo and in vitro together with parallel electrophysiological studies, employed to assess ongoing change within both these axotomized neurons as well as those neurons revealing no evidence of axotomy and remaining intact. In companion with these studies, targeted therapeutic strategies will be used, together with knock-out approaches to assess their effect upon the progression of traumatically-induced axonal change and its electrophysiological sequelae. Additionally, the overlying cerebral microcirculation related to these sites of axonal injury will be assessed to determine if mild TBI impairs their reactivity to known physiological challenges. These issues will be addressed not only in the context of mild TBI uncomplicated by secondary insult but also, they will be reevaluated in the context of repeated mild TBI to determine if the repeat injury exacerbates any observed structural or functional change. We will also explore if repeat injury precipitates an enduring cascade of microvascular abnormalities that lead to sustained vasodilation, lack of vascular responsivity, elevated intracranial pressure, and subsequent reduction of cerebral perfusion pressure. Through the conduct of the studies proposed we believe that we will provide unprecedented insight into the initiating pathogenesis of traumatically-induced axonal injury and its potential therapeutic modification. Similarly, the proposed electrophysiological studies should provide unique insight into the neurophysiological sequelae of mild TBI, revealing alterations not only in the axotomized populations but also in the related non-axotomized/intact neuronal population. Lastly, through the use of repeated injuries, we hope to provide critical insight into the pathobiology underlying the significant morbidity associated with this condition, which is a major confound of sporting-related injury. In our estimation, the studies proposed are not only descriptive but also mechanistic, therapeutic, and translational, thereby providing a relatively sophisticated platform for addressing some of the most complex issues currently confounding our understanding of mild TBI. PUBLIC HEALTH RELEVANCE: The proposed studies are highly relevant to public health in that they address a national healthcare problem focusing on traumatic brain injury and its exacerbation by repeated injury. Further benefit will follow from the conduct of the proposed study through its attempts to better understand the pathogenesis of traumatically- induced axonal damage and its targeted therapeutic attenuation.